Soldering is a commonly employed method for mechanically and electrically bonding a lead of an electronic component or a connector to a metallized area on a substrate, such as a circuit board. A typical soldering operation is carried out by heating the lead and the metallized area in contact therewith, and then applying solder so that the solder melts (i.e., reflows) and coats the lead and metallized area. The solder, upon solidification, electrically and mechanically bonds the lead to the metallized area. Prior to actually carrying out a soldering operation, it is often useful to apply soldering flux to the lead and/or to the metallized area. The advantage obtained in applying soldering flux is that the flux acts both to clean and to promote solder wetting to the metallized area and lead, thereby enhancing the quality of the resultant soldering bond.
The disadvantage with most types of solder flux is that the flux leaves a post-solder residue. To obtain a good cosmetic appearance, and to assure good electrical testability of the circuit board, as well as to remove residues which are sometimes corrosive, the circuit board is usually cleaned after soldering. Cleaning is a non-value-added operation and, often, an environmental concern. For these reasons much effort is currently being devoted to developing soldering processes which obviate the need to clean.
One recent approach to reducing the incidence of residues in connection with a manually performed soldering operation is to employ so-called "low-solids core" solder which is comprised of hollow tin-lead solder wire whose core is filled with a low-solids flux. The low-solids flux within such core solder contains a reduced volume of flux solids which yields a reduced amount of flux residue following soldering. The disadvantage of low-solids core solder is that the flux contained within the solder tends to be more chemically active than conventional rosin-based fluxes, and hence the residues tend to be more corrosive. While the corrosion problem associated with the use of low-solids core solder can be obviated by solder-coating the metallized areas prior to soldering the leads thereto, the need to solder coat the metallized areas increases the overall fabrication cost, which is undesirable.
Thus, there is a need for a soldering technique which reduces the amount of flux residues, and hence the corrosion problem, when manually soldering a lead to a bare (i.e., non-solder-coated) metallized area on a substrate with a flux-containing (e.g., core) solder.